RFID and distance measurement

Hello Guys,

I have a new project that I want use RFID to verify ID of item and I need measurement distance from reader to TAG. The maximum distance between the reader to TAG is about 7 cm and I want know distance between reader to TAG with accuracy about 1 cm.

Is it possible?

Maybe an ultrasonic sensor will work, if someone is holding the rfid tag it will detect their hand as well.

The return magnetic field variation from the rfid tag varies a lot depending on the orientation of the tag relative to the reader, it works best with the reader coil perpendicular to the reader coil.

Theoretically yes, but quite difficult in practice, if you solely depend on RFID without using any additional sensor (e.g. ultrasonic sensor mentioned by throwaway18).

Considering the close operation distance (max 7 cm), you are working in the near field, no matter you use LF, HF or UHF RFID band. You need to carefully study the relationship between the distance and some performance parameter (e.g. signal strength, signal phase). Such a relationship enables predicting the distance between the tag and the reader antenna/coil. Unfortunately the relationship in the near field is usually complicated and sensitive to any disturbance (e.g. the tag orientation mentioned by throwaway18).

Tnx for your help,

This project will be final work to my graduation, so I want use only RFID to solve my problem by research questions.
Being more specific, I want identificate if item was placed on shelf in correct position. If worker put item out of position, the system will identify with RFID but in wrong position.

I worked with LF RFID from Texas Instruments but it not have RSSI information. Can I implement hardware to check it or exist any similar RFID with RSSI ?

Tnx very much

F-Moura said:

Can I implement hardware to check it or exist any similar RFID with RSSI ?

Click to expand...

According to this article, distance measurement based on RSSI, gives unreliable results:

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.454.3357&rep=rep1&type=pdf

Absolutely, you can applying RSSI to measure the distance between Reader and Tag, but I am not sure the result can be accepted or not. I tried detecting movement using RSSI and the result can be acceptable.

If your accuracy tolerance is 1/7th of the range, and the tag orientation is consistent, then it should be achieveable.

But considering peak to null ratio of the RFID tag radiation pattern from orientation, you would need diversity control with multiple antenna for the reader.

If you just want to know whether the item is placed on the correct position on the shelf, maybe you don't need to evoke RSSI. I have little experience with LF RFID systems, so I try to explain with UHF RFID systems.

Almost all fixed-station UHF RFID readers provide multiple antenna ports (up to 32 ports). Suppose you connect 8 antennas to a UHF RFID reader, and a tag is detected by one of the 8 antennas, the port number of the antenna which detects the tag would be reported together with the tag ID. Based on the port number reported, you can have an idea of the location of the item which the tag is attached to - the item should be within the read zone of the specified antenna. If you have a quite limited read range for the specified antenna (e.g. you use a near-field antenna), you can even say the item is directly located on the antenna.

The drawback of the approach mentioned above is that the location resolution depends on the antenna size and that the implementation consumes a lot of antennas. A more advanced approach attempts to locate the item/tag with a sub-antenna resolution - to determine which part of the antenna the item/tag is at. Mostly this requires using complex antennas. For example, you need to design a feeding network to excite different elements of an array consecutively. Or you can utilize a phased array.